TiO2 and Al2O3 coating and corrosion of magnesium and its alloys by plasma spray technique

Magnesium and its alloys are very low density materials, but, they have poor mechanical and chemical properties. Successful coating of these materials, which have very low corrosion resistance, will increase their corrosion resistance. There will also be a change in the mechanical properties and the use of Mg and its alloys in more aggressive environments will be expanded. The aim of this study is to investigate the ability of coating of Pure Mg, AZ31 and AZ91 Mg alloys with Al2O3 and TiO2 using the plasma spray coating method. Experimental studies results showed that, pure Mg, AZ31 and AZ91 Mg alloys were successfully coated with Al2O3 and TiO2 based ceramics by plasma spraying technique. As a result of the examination of the cross-sectional microstructures, a continuous adhesion to the substrate was obtained for all coatings on the AZ91 alloy, except for the TiO2 coating. The cross-sectional microstructures have the characteristics of conventional plasma spray ceramic coatings and contain little porosity and capillary cracks. When the phase structures of Al2O3 and TiO2 based materials were examined before and after coating, there was no phase transformation occurred in TiO2 after plasma sputtering, while crystallinity decreased in Al2O3 and Al2O3+TiO2 ceramics. The high plasma enthalpy and rapid cooling rate during the coating production caused the crystalline phases in the coating structure to turn into amorphous. In the adhesion tests performed according to the VDI 3198 standard, it was understood that all AZ31 coatings exhibited an acceptable adhesion. According to the electrochemical corrosion tests performed in 3.5% NaCl environment, all coatings produced a lower corrosion rate than that of the uncoated backing material. While the lowest corrosion rate was measured for Al2O3+ %3TiO2 coating, the highest corrosion rate was measured for TiO2 coating. When the experimental results obtained were examined in general, it was understood that plasma sprayed ceramic-based coatings were effective in the protection of Mg and its alloys from corrosion